If a dielectric slab of dielectric constant 3 is introduced between the plates of a capacitor having electric field $15 \pi \mathrm{NC}^{-1}$, then the electric displacement is

Four capacitors are connected as shown in the figure. If $C_1, C_2, C_3$ and $C_4$ are in the ratio of $1: 2: 3: 4$, then the ratio of the charges on the capacitors $C_2$ and $C_4$ is

A capacitor of capacitance $2 \mu \mathrm{~F}$ is charged with the help of a 60 V battery. After disconnecting the battery, if this capacitor is connected in parallel with another uncharged capacitor of capacitance $l \mu \mathrm{~F}$, then the potential difference across the plates of $2 \mu \mathrm{~F}$ capacitor is

The energy stored in a capacitor of capacitance $10 \mu \mathrm{~F}$ when charged to a potential of 6 kV is